The use of fossil fuels for power generation and in the petrochemical industry is expected still to increase in the first decades of the next century. The demand for low-sulfur fossil fuels has been intensified by the increasing regulatory standards for reduced levels of sulfur-oxides in atmospheric emissions, by the decline of easily accessible sources of conventional and light crude oils, and by the high cost of physicochemical process of hydrodesulfurization (HDS). It can be estimated that in the next decades 30% of oil should be desulfurized.
The use of microorganisms for the biodesulfurization of high sulfur coals and oil has been proposed as an interesting alternative for the reduction of the organosulfur contentof fossil fuels [Monticello Finnerty, Ann. Rev. Microbiol. 39 (1985) 371.; Bhadra et al., Biotechnol. Adv. 5 (1987) 1.; Kilbane and Jackowski, Biotechnol. Bioeng. 40 (1992) 1107]. Selective sulfur removal has also been reported by a pathway involving the conversion of dibenzothiophene (DBT) to 2-hydroxybiphenyl (2-HBP) and sulfate, as in the case of Corynebacterium sp. [Omori et al., Appl. Environ. Microbiol. 49 (1992) 911.], Rhodococcus erythropolis [Izumi et al., Appl. Environ. Microbiol. 60 (1994) 223.; Ohshiro et al., Appl. Microbiol. Biotechnol. 44 (1995) 249.; Wang et al., Appl. Environ. Mircobiol. 62 (1996) 3066.; Wang and Krawiec, Appl. Environ. Mircobiol. 62 (1996) 1670.], and Rhodococcus sp. strain IGTS8 [Kilbane and Jackowski, Biotechnol. Bioeng. 40 (1992) 1107.; Kayser et al., J. Gen. Mircobiol. 139 (1993) 3123.]. Most of the microbial biodesulfurization studies have focused on the aerobic conversion of DBT, coal or fuels. Nevertheless, reductive desulfurization of fossil fuels is an idea proposed more than 25 years ago by Kurita et al. [Denis-Larose et al., Appl. Environ. Mircobiol. 63 (1997) 2915.]. Mixed cultures containing sulfate-reducing bacteria (SBR) desulfurized a variety of model compounds, including thiophenes, organosulfides and petroleum preparations [Kxc3x6hler et al., Zentralb1. Mikrobiol. 139 (1984) 239.; Miller, Appl. Environ. Microbiol. 58 (1992) 2176.; Eckart et al., Zentralb1. Mikrobiol. 141 (1986) 291.]. Reductive desulfurization of DBT to form hydrogen sulfide and biphenyl has been achieved by several species of SRB that are able to grow using DBT as sole source of sulfur and sole electron acceptor [Kim et al., Biotechnol. Lett. 12 (1990) 761.; Kim et al., Fuel Process. Technol. 43 (1995) 87.]. Hydrogen gas is the normal source of reducing equivalent, however, electrochemically generated reducing equivalents can be incorporated into the normal electron transport system of SRB.
Microbial desulfurization of petroleum derivatives has two main problems: Microbial activity is carried out in aqueous phase and under mild conditions, thus a two phase system reactor with the intrinsic mass transfer limitations would be needed to metabolize the hydrophobic substrate. On the other hand, the microbial biocatalyst must have a broad substrate specificity for the various organosulfur compound present in oil.
These problems could be addressed by using broad specificity enzyes instead of whole microorganisms. Enzymes are able to perform catalytic reactions in organic solvents [Dordick, Enzyme Microb. Technol. 11 (1989) 194.], in which the mass transfer limitations are reduced. The solvent could be the fuel itself. Under anhydrous conditions or at very low water activity, enzymes are generally more thermostable, and reactions could be performed at temperatures higher than 100xc2x0 C. [Mozhaev et al., FEBS Lett. 292 (1991) 159.]. Biocatalytic modification of complex mixtures from petroleum, such as asphaltenes, have been performed in organic solvents for removal of metals [Fedorak et al., Enzyme Microb. Technol. 15 (1993) 429.].
Therefore, it is desirable to develop a biotechnological process which will remove sulfur-containing compound from fossil fuels in one-phase and non-aqueous system.
The invention relates to a method of removing organosulfur compounds from a fossil fuel comprising two steps. First, the contact of a fossil fuel with a biocatalyst, comprising peroxidases and other hemoproteins, which under suitable conditions oxidizes thiophenes and organosulfides to their respective sulfoxides and sulfones, and a second step in which the oxidized compounds can be separated by a distillation process or an other physicochemical process. The preferred systems included non aqueous systems such as water-saturated fuel, fuel solutions in organic solvents or in other petroleum derivatives. The biocatalyst could be free or immobilized in a support. Preferred embodiments of the biocatalyst include chloroperoxidase from Caldariomyces fumago, type-c cytochromes, or other hemoproteins from animal, plant or microbial cells. In one preferred embodiment of the invention, the oxidized organosulfur compounds are separated from the fuel by distillation, resulting in a low sulfur content stream.